1. Field of the Invention
The present invention generally relates to an anti-lock brake control system for preventing lockup of vehicle wheel brakes during braking of the vehicle, and more particularly, to an anti-lock modulator constituting part of the anti-lock brake control system and provided with a housing structure accommodating flow control valves, discharge valves and plunger pumps.
2. Description of the Prior Art
Nowadays, automobile vehicles except relatively small vehicles are generally provided with an anti-lock brake control system. There is an increased demand for a small-sized system which can be applied to a relatively small automobile vehicle and manufactured at a low cost.
The Japanese Patent Publication No. 49-28307 discloses an anti-lock brake control system in which two solenoid valves are provided for each of four wheels for controlling brake pressure in three modes, i.e., pressure reducing, holding and increasing modes.
The British Patent Publication No. 851260 discloses another anti-lock brake control system which can be formed into a compact size at a reduced cost as compared with the aforementioned system. In this system, the anti-lock brake control is done in two modes i.e., pressure reducing and gradually increasing modes using a single solenoid valve and a flow control valve, which is not operated by a solenoid, for each wheel.
FIGS. 1 to 3 depict an anti-lock modulator for use in the system mentioned above, which is provided with two spaced housings 1 and 2. The upper housing 1 accommodates a flow control valve 3 while the lower housing 2 accommodates a plunger pump 4. A solenoid-operated discharge valve 5 is sandwiched between these two housings 1 and 2. A reservoir 8 accommodating a piston 6 and a compression spring 7 for biasing the piston 6 is generally provided between the discharge valve 5 and the plunger pump 4 in the direction of flow of working fluid. In this modulator, the reservoir 8 is formed in a lower portion of the lower housing 2 and closed by a plug 9.
As described above, the conventional modulator as shown in FIGS. 1 to 3 is comprised of the two spaced housings 1 and 2, the flow control valve 3 and the plunger pump 4 accommodated in these two housings 1 and 2, respectively, and the discharge valve 5 sandwiched between the housings 1 and 2. Accordingly, a fluid passage is necessarily required for allowing the communication of the working fluid between the spaced two housings 1 and 2. To this end, a pipe 10 is occasionally employed which is securely sandwiched between the two housings 1 and 2. However, this kind of modulator is disadvantageous in size, cost, reliability or the like.
Moreover, since the reservoir 8 needs the plug 9, not only the number of parts but assembling processes increase, resulting disadvantageously in an increased cost.